Thin film transistors (TFTs) are widely used as a pixel switching element for display apparatuses such as a liquid crystal display and an EL display. Moreover, in recent years, there are a growing number of examples in which the driver circuit of a pixel array is also formed of TFTs on the same substrate. Conventionally, such TFTs used to be fabricated on a glass substrate by using amorphous or polycrystalline silicon. However, a problem exists in that CVD apparatuses which are used for the fabrication of TFTs by use of silicon are very expensive and the area enlargement of a display apparatus etc. which uses TFTs will result in a significant increase in production cost. Moreover, since the process of forming a film of amorphous or polycrystalline silicon is performed at a very high temperature, there are restrictions such as that the materials which can be used as the substrate are limited, and light-weight resin substrates etc. cannot be used.
A carbon nanotube (hereafter, abbreviated as “CNT”) is tubular carbon molecules consisting of carbon alone, and has a structure of a rolled-up Graphene sheet made up of six-membered rings of carbon. A CNT which is formed by rolling up a single Graphene sheet into a tubular form is called a “single-walled nanotube” (hereafter, abbreviated as SWNT) and CNTs formed by laminating a plurality of layers of tubular CNTs having different diameters are called a “multi-walled nanotube” (hereinafter, abbreviated as MWNT). The diameter of a SWNT is about 1 nm and that of a MWNT is about several tens of nm. Among CNTs, depending on the difference in the direction of rolling up the Graphene sheet, that is, the difference in the orientation of the six-membered rings of carbon atoms with respect to the circumferential direction, other than the difference in diameter, there are various CNTs of different chiralities such as, for example, a spiral CNT, a zigzag CNT, or an armchair-type CNT. Both metallic and semiconductive properties manifest in SWNTs due to the difference in chirality.
By growing SWNTs having features as describe above randomly between a source and drain electrodes by, for example, a chemical vapor deposition (CVD) method, it is possible to fabricate a field-effect transistor of which channel layer is comprised of SWNTs. Further, this channel layer comprised of SWNTs can also be formed by dispersing CNTs in a liquid, and coating-depositing, and printing them on a substrate.
Non-Patent Document 1 reports that in a thus-formed random network of CNTs, many contacts are formed and connections among carbon nanotubes take place, which can be utilized for the channel layer of a thin film transistor. According to above-described Non-Patent Document 1, when the density of single-walled carbon nanotube in the channel layer is around 1/μm2, it is possible to obtain an on/off ratio of 5 orders of magnitude and a mobility of 7 cm2/Vs, and fabricate a good thin film transistor.
A random network of CNTs can be obtained by coating or printing a dispersion of CNT as described above. This process can realize device area enlargement at low cost, and has a low process temperature thus contributing less restriction on the selection of materials to be used as the substrate. Therefore, compared with a silicon-based TFT formed on a glass substrate which has been conventionally used, it is possible to significantly suppress the production cost. In recent years, there are frequent reports on TFTs which use random networks of CNT and, for example, there are reports of Non-Patent Documents 2 to 4.    Non-Patent Document 1: E. S. Snow et al., Applied Physics Letters, vol. 82, p. 2145, (2003).    Non-Patent Document 2: E. Artukovic, M. Kaempgen, D. S. Hecht, S. Roth, G. Gruner, Nano Letters vol. 5, p. 757, (2005).    Non-Patent Document 3: S.-H. Hur, 0.0. Park, J. A. Rogers, Applied Physics Letters, vol. 86, p. 243502 (2005).    Non-Patent Document 4: T. Takenobu, T. Takahashi, T. Kanbara, K. Tsukagoshi, Y. Aoyagi, Y. Iwasa, Applied Physics Letters, vol. 88, p. 33511, (2006).